Managed model instances

We should introduce "managed model isntances", i.e. objects that are automatically created and thus managed by some app. Examples include:

• Notifications (notifications actually already support this)
• Group types (thinking of Chronos making a "course" group type that it uses for course groups); by doing so, it can reliably find groups that are managed in the timetable, and link semantics to it
• Objects in Chronos that are managed by an Untis import

Apps can then decide what to do with managed instances, with options including hiding managed objects from the UI, or other behaviour to make sure managed objects do not fill up the user-visible interfaces.